Cooling element architecture for mems-based cooling system architecture

ABSTRACT

A cooling system including a support structure and a cooling element is described. The cooling element has a thickness and includes an anchored region and a cantilevered arm. The anchored region is coupled to and supported by the support structure. The cantilevered arm extends outward from the anchored region. The cantilevered arm includes at least one cavity therein. The at least one cavity has a depth of at least one-third and not more than three-fourths of the thickness of the cooling element. The cooling element is configured to undergo vibrational motion when actuated to drive a fluid for cooling a heat-generating structure.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 63/220,862 entitled PIEZOELECTRIC ACTIVE MEMS COOLING SYSTEMSINCLUDING ENGINEERED ACTUATORS, TAILORED ORIFICES, CONTROLLED GAPS, ANDSTRIP LEVEL MANUFACTURING filed Jul. 12, 2021 and U.S. ProvisionalPatent Application No. 63/353,490 entitled MEMS VIBRATIONAL COOLINGSYSTEM HAVING AN INTEGRATED SPOUT AND A GRAPHITE COVER filed Jun. 17,2022, both of which are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

As computing devices grow in speed and computing power, the heatgenerated by the computing devices also increases. Various mechanismshave been proposed to address the generation of heat. Active devices,such as fans, may be used to drive air through large computing devices,such as laptop computers or desktop computers. Passive cooling devices,such as heat spreaders, may be used in smaller, mobile computingdevices, such as smartphones, virtual reality devices and tabletcomputers. However, such active and passive devices may be unable toadequately cool both mobile devices such as smartphones and largerdevices such as laptops and desktop computers. Moreover, incorporatingcooling solutions into computing devices may be challenging.Consequently, additional cooling solutions for computing devices aredesired.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIGS. 1A-1G depict an embodiment of an active MEMS cooling systemincluding a centrally anchored cooling element.

FIGS. 2A-2B depict an embodiment of an active MEMS cooling systemincluding a centrally anchored cooling element.

FIGS. 3A-3E depict an embodiment of an active MEMS cooling system formedin a tile.

FIGS. 4A-4D depict embodiments of active MEMS cooling systems includingcentrally anchored cooling elements.

FIGS. 5A-5B depict embodiments of active MEMS cooling systems includingcentrally anchored cooling elements.

FIG. 6 depicts an embodiment of a method for using an active coolingmems system.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

As semiconductor devices become increasingly powerful, the heatgenerated during operations also grows. For example, processors formobile devices such as smartphones, tablet computers, notebookcomputers, and virtual reality devices as well as for other computingdevices such as servers, can operate at high clock speeds, but produce asignificant amount of heat. Because of the quantity of heat produced,processors may run at full speed only for a relatively short period oftime. After this time expires, throttling (e.g. slowing of theprocessor's clock speed) occurs. Although throttling can reduce heatgeneration, it also adversely affects processor speed and, therefore,the performance of devices using the processors. As technology moves to5G and beyond, this issue is expected to be exacerbated. Further, othercomponents in a computing device may generate heat. Thus, thermalmanagement is increasingly an issue for computing devices.

Larger computing devices, such as laptop computers, desktop computers,or servers, include active cooling systems. Active cooling systems arethose in which an electrical signal is used to drive cooling. Anelectric fan that has rotating blades is an example of an active coolingsystem, while a heat sink is an example of a passive cooling system.When energized, the fan's rotating blades drive air through the largerdevices to cool internal components. However, space and otherlimitations in computing devices limit the use of active coolingsystems. Fans are typically too large for mobile and/or thinner devicessuch as smartphones and tablet or notebook computers. Fans also may havelimited efficacy because of the boundary layer of air existing at thesurface of the components, because they provide a limited airspeed forair flow across the hot surface desired to be cooled, and because theymay generate an excessive amount of noise. Fans also have a limitedbackpressure. Space and power limitations may further restrict theability to provide electrical connection to active cooling systems. Forexample, if multiple active cooling systems are used, the connections tothe active cooling systems may be required to fit within a small area.In addition, the power consumed by such a cooling system may be desiredto be small, particularly for mobile devices. Consequently, activecooling systems face particular challenges when used in computingdevices such as active computing devices. Passive cooling solutions mayinclude components such as a heat spreader and a heat pipe or vaporchamber to transfer heat to a heat exchanger. Although a heat spreadersomewhat mitigates the temperature increase at hot spots, the amount ofheat produced in current and future devices may not be adequatelyaddressed. Similarly, a heat pipe or vapor chamber may provide aninsufficient amount of heat transfer to remove excessive heat generated.Thus, additional cooling solutions are desired.

A cooling system including a support structure, a cooling element, and abottom plate is described. The cooling element has a central region anda perimeter. The cooling element is supported by the support structureat the central region. At least a portion of the perimeter is unpinned.The cooling element undergoes vibrational motion when actuated to drivea fluid toward a heat-generating structure. The bottom plate hasorifices and at least one cavity therein. The at least one cavity isadjacent to and fluidically connected with the orifices. The at leastone cavity and the orifices define an orifice distance between theorifices and the heat-generating structure and an orifice length withinthe bottom plate. In some embodiments, the orifice distance is at leasttwo hundred micrometers. The orifice length may be at least one hundredand fifty micrometers and not more than three hundred micrometers. Theheat-generating structure and the bottom plate define a gap between aportion of the bottom plate and a portion of the heat-generatingstructure. In some embodiments, the gap is at least two hundred micronsand not more than five hundred microns high. In some such embodiments,the gap is at least three hundred and fifty microns and not more thanfour hundred and fifty hundred microns high.

In some embodiments, the cavity/cavities include a first cavity on afirst side of the orifices and/or a second cavity on a second side ofthe orifices. The first cavity is further from the heat-generatingstructure than the orifices. The second cavity is closer to theheat-generating structure than the orifices. The bottom plate mayinclude or consist of a material having low internal losses. Forexample, the material may include at least one of Al7075, Al6063, Ti64,Ti Grade 2, Ti Grade 9, a beryllium-copper alloy, Monel, aluminumbronze, aluminum, SUS 304, or SUS316. In some embodiments, the bottomplate includes an orifice plate and a jet channel plate. The jet channelplate is between the orifice plate and the heat-generating structure.The heat-generating structure may have a depression therein. In suchembodiments, the orifice distance includes a height of the depression.

In some embodiments the cooling system includes a plurality of coolingcells. Each cell may include the support structure, cooling element,bottom plates, and/or other structures described herein. A method ofcooling a heat-generating structure is also described. In someembodiments, the method includes driving cooling element(s) in thesystems described herein. In some such embodiments, the coolingelement(s) are driven at or near resonance.

FIGS. 1A-1G are diagrams depicting an exemplary embodiment of activeMEMS cooling system 100 usable with heat-generating structure 102 andincluding a centrally anchored cooling element 120 or 120′. Coolingelement 120 is shown in FIGS. 1A-1F and cooling element 120′ is shown inFIG. 1G. For clarity, only certain components are shown. FIGS. 1A-1G arenot to scale. FIGS. 1A and 1B depict cross-sectional and top views ofcooling system 100 in a neutral position. FIGS. 1C-1D depict coolingsystem 100 during actuation for in-phase vibrational motion. FIGS. 1E-1Fdepict cooling system 100 during actuation for out-of-phase vibrationalmotion. Although shown as symmetric, cooling system 100 need not be.

Cooling system 100 includes top plate 110 having vent 112 therein,cooling element 120, orifice plate 130 having orifices 132 therein,support structure (or “anchor”) 160 and chambers 140 and 150(collectively chamber 140/150) formed therein. Cooling element 120 issupported at its central region by anchor 160. Regions of coolingelement 120 closer to and including portions of the cooling element'sperimeter (e.g. tip 121) vibrate when actuated. In some embodiments, tip121 of cooling element 120 includes a portion of the perimeter furthestfrom anchor 160 and undergoes the largest deflection during actuation ofcooling element 120. For clarity, only one tip 121 of cooling element120 is labeled in FIG. 1A. Also shown is pedestal 190 that connectsorifice plate 130 to and offsets orifice plate 130 from heat-generatingstructure 102. In some embodiments, pedestal 190 also thermally couplesorifice plate 130 to heat-generating structure 102. In some embodiments,an additional jet channel plate may be present and supported by pedestal190. Thus orifice plate 130 and/or such a jet channel plate may be partor all of a bottom plate supported by pedestal 190. Thus, multipleplates and/or plate(s) having various structures may be used at thebottom plate for cooling system 100.

FIG. 1A depicts cooling system 100 in a neutral position. Thus, coolingelement 120 is shown as substantially flat. For in-phase operation,cooling element 120 is driven to vibrate between positions shown inFIGS. 1C and 1D. This vibrational motion draws fluid (e.g. air) intovent 112, through chambers 140 and 150 and out orifices 132 at highspeed and/or flow rates. For example, the speed at which the fluidimpinges on heat-generating structure 102 may be at least thirty metersper second. In some embodiments, the fluid is driven by cooling element120 toward heat-generating structure 102 at a speed of at leastforty-five meters per second. In some embodiments, the fluid is driventoward heat-generating structure 102 by cooling element 120 at speeds ofat least sixty meters per second. Other speeds may be possible in someembodiments. Cooling system 100 is also configured so that little or nofluid is drawn back into chamber 140/150 through orifices 132 by thevibrational motion of cooling element 120.

Heat-generating structure 102 is desired to be cooled by cooling system100. In some embodiments, heat-generating structure 102 generates heat.For example, heat-generating structure may be an integrated circuit. Insome embodiments, heat-generating structure 102 is desired to be cooledbut does not generate heat itself. Heat-generating structure 102 mayconduct heat (e.g. from a nearby object that generates heat). Forexample, heat-generating structure 102 might be a heat spreader or avapor chamber. Thus, heat-generating structure 102 may includesemiconductor component(s) including individual integrated circuitcomponents such as processors, other integrated circuit(s) and/or chippackage(s); sensor(s); optical device(s); one or more batteries; othercomponent(s) of an electronic device such as a computing device; heatspreaders; heat pipes; other electronic component(s) and/or otherdevice(s) desired to be cooled. In some embodiments, heat-generatingstructure 102 may be a thermally conductive part of a module containingcooling system 100. For example, cooling system 100 may be affixed toheat-generating structure 102, which may be coupled to another heatsink, vapor chamber, integrated circuit, or other separate structuredesired to be cooled.

The devices in which cooling system 100 is desired to be used may alsohave limited space in which to place a cooling system. For example,cooling system 100 may be used in computing devices. Such computingdevices may include but are not limited to smartphones, tabletcomputers, laptop computers, tablets, two-in-one laptops, hand heldgaming systems, digital cameras, virtual reality headsets, augmentedreality headsets, mixed reality headsets and other devices that arethin. Cooling system 100 may be a micro-electro-mechanical system (MEMS)cooling system capable of residing within mobile computing devicesand/or other devices having limited space in at least one dimension. Forexample, the total height, h3, of cooling system 100 (from the top ofheat-generating structure 102 to the top of top plate 110) may be lessthan 3 millimeters. In some embodiments, the total height of coolingsystem 100 is less than 2.5 millimeters. In some embodiments, the totalheight of cooling system 100 (from the top of heat-generating structure102 to the top of top plate 110) may be less than 2 millimeters. In someembodiments, the total height of cooling system 100 is not more than 1.5millimeters. In some embodiments, this total height is not more than 1.1millimeters. In some embodiments, the total height does not exceed onemillimeter. In some embodiments, the total height does not exceed twohundred and fifty micrometers. Similarly, the distance between thebottom of orifice plate 130 and the top of heat-generating structure102, y, may be small. In some embodiments, y is at least two hundredmicrometers and not more than 1.2 millimeter. For example, y may be atleast two hundred and fifty micrometers and not more than three hundredmicrometers. In some embodiments, y is at least five hundred micrometersand not more than one millimeter. In some embodiments, y is at least twohundred micrometers and not more than three hundred micrometers. Thus,cooling system 100 is usable in computing devices and/or other deviceshaving limited space in at least one dimension. However, nothingprevents the use of cooling system 100 in devices having fewerlimitations on space and/or for purposes other than cooling. Althoughone cooling system 100 is shown (e.g. one cooling cell), multiplecooling systems 100 might be used in connection with heat-generatingstructure 102. For example, a one or two-dimensional array of coolingcells might be utilized.

Cooling system 100 is in communication with a fluid used to coolheat-generating structure 102. The fluid may be a gas or a liquid. Forexample, the fluid may be air. In some embodiments, the fluid includesfluid from outside of the device in which cooling system 100 resides(e.g. provided through external vents in the device). In someembodiments, the fluid circulates within the device in which coolingsystem 100 resides (e.g. in an enclosed device).

Cooling element 120 can be considered to divide the interior of activeMEMS cooling system 100 into top chamber 140 and bottom chamber 150. Topchamber 140 is formed by cooling element 120, the sides, and top plate110. Bottom chamber 150 is formed by orifice plate 130, the sides,cooling element 120 and anchor 160. Top chamber 140 and bottom chamber150 are connected at the periphery of cooling element 120 and togetherform chamber 140/150 (e.g. an interior chamber of cooling system 100).

The size and configuration of top chamber 140 may be a function of thecell (cooling system 100) dimensions, cooling element 120 motion, andthe frequency of operation. Top chamber 140 has a height, h1. The heightof top chamber 140 may be selected to provide sufficient pressure todrive the fluid to bottom chamber 150 and through orifices 132 at thedesired flow rate and/or speed. Top chamber 140 is also sufficientlytall that cooling element 120 does not contact top plate 110 whenactuated. In some embodiments, the height of top chamber 140 is at leastfifty micrometers and not more than five hundred micrometers. In someembodiments, top chamber 140 has a height of at least two hundred andnot more than three hundred micrometers.

Bottom chamber 150 has a height, h2. In some embodiments, the height ofbottom chamber 150 is sufficient to accommodate the motion of coolingelement 120. Thus, no portion of cooling element 120 contacts orificeplate 130 during normal operation. Bottom chamber 150 is generallysmaller than top chamber 140 and may aid in reducing the backflow offluid into orifices 132. In some embodiments, the height of bottomchamber 150 is the maximum deflection of cooling element 120 plus atleast five micrometers and not more than ten micrometers. In someembodiments, the deflection of cooling element 120 (e.g. the deflectionof tip 121), z, has an amplitude of at least ten micrometers and notmore than one hundred micrometers. In some such embodiments, theamplitude of deflection of cooling element 120 is at least tenmicrometers and not more than sixty micrometers. However, the amplitudeof deflection of cooling element 120 depends on factors such as thedesired flow rate through cooling system 100 and the configuration ofcooling system 100. Thus, the height of bottom chamber 150 generallydepends on the flow rate through and other components of cooling system100.

Top plate 110 includes vent 112 through which fluid may be drawn intocooling system 100. Top vent 112 may have a size chosen based on thedesired acoustic pressure in chamber 140. For example, in someembodiments, the width, w, of vent 112 is at least five hundredmicrometers and not more than one thousand micrometers. In someembodiments, the width of vent 112 is at least two hundred fiftymicrometers and not more than two thousand micrometers. In theembodiment shown, vent 112 is a centrally located aperture in top plate110. In other embodiments, vent 112 may be located elsewhere. Forexample, vent 112 may be closer to one of the edges of top plate 110.Vent 112 may have a circular, rectangular or other shaped footprint.Although a single vent 112 is shown, multiple vents might be used. Forexample, vents may be offset toward the edges of top chamber 140 or belocated on the side(s) of top chamber 140. Although top plate 110 isshown as substantially flat, in some embodiments trenches and/or otherstructures may be provided in top plate 110 to modify the configurationof top chamber 140 and/or the region above top plate 110.

Anchor (support structure) 160 supports cooling element 120 at thecentral portion of cooling element 120. Thus, at least part of theperimeter of cooling element 120 is unpinned and free to vibrate. Insome embodiments, anchor 160 extends along a central axis of coolingelement 120 (e.g. perpendicular to the page in FIGS. 1A-1F). In suchembodiments, portions of cooling element 120 that vibrate (e.g.including tip 121) move in a cantilevered fashion. Thus, portions ofcooling element 120 may move in a manner analogous to the wings of abutterfly (i.e. in phase) and/or analogous to a seesaw (i.e. out ofphase). Thus, the portions of cooling element 120 that vibrate in acantilevered fashion do so in phase in some embodiments and out of phasein other embodiments. In some embodiments, anchor 160 does not extendalong an axis of cooling element 120. In such embodiments, all portionsof the perimeter of cooling element 120 are free to vibrate (e.g.analogous to a jellyfish). In the embodiment shown, anchor 160 supportscooling element 120 from the bottom of cooling element 120. In otherembodiments, anchor 160 may support cooling element 120 in anothermanner. For example, anchor 160 may support cooling element 120 from thetop (e.g. cooling element 120 hangs from anchor 160). In someembodiments, the width, a, of anchor 160 is at least 0.5 millimeters andnot more than four millimeters. In some embodiments, the width of anchor160 is at least two millimeters and not more than 2.5 millimeters.Anchor 160 may occupy at least ten percent and not more than fiftypercent of cooling element 120.

Cooling element 120 has a first side distal from heat-generatingstructure 102 and a second side proximate to heat-generating structure102. In the embodiment shown in FIGS. 1A-1F, the first side of coolingelement 120 is the top of cooling element 120 (closer to top plate 110)and the second side is the bottom of cooling element 120 (closer toorifice plate 130). Cooling element 120 is actuated to undergovibrational motion as shown in FIGS. 1A-1F. The vibrational motion ofcooling element 120 drives fluid from the first side of cooling element120 distal from heat-generating structure 102 (e.g. from top chamber140) to a second side of cooling element 120 proximate toheat-generating structure 102 (e.g. to bottom chamber 150). Thevibrational motion of cooling element 120 also draws fluid through vent112 and into top chamber 140; forces fluid from top chamber 140 tobottom chamber 150; and drives fluid from bottom chamber 150 throughorifices 132 of orifice plate 130. Thus, cooling element 120 may beviewed as an actuator. Although described in the context of a single,continuous cooling element, in some embodiments, cooling element 120 maybe formed by two (or more) cooling elements. Each of the coolingelements is depicted as one portion pinned (e.g. supported by supportstructure 160) and an opposite portion unpinned. Thus, a single,centrally supported cooling element 120 may be formed by a combinationof multiple cooling elements supported at an edge.

Cooling element 120 has a length, L, that depends upon the frequency atwhich cooling element 120 is desired to vibrate. In some embodiments,the length of cooling element 120 is at least four millimeters and notmore than ten millimeters. In some such embodiments, cooling element 120has a length of at least six millimeters and not more than eightmillimeters. The depth of cooling element 120 (e.g. perpendicular to theplane shown in FIGS. 1A-1F) may vary from one fourth of L through twiceL. For example, cooling element 120 may have the same depth as length.The thickness, t, of cooling element 120 may vary based upon theconfiguration of cooling element 120 and/or the frequency at whichcooling element 120 is desired to be actuated. In some embodiments, thecooling element thickness is at least two hundred micrometers and notmore than three hundred and fifty micrometers for cooling element 120having a length of eight millimeters and driven at a frequency of atleast twenty kilohertz and not more than twenty-five kilohertz. Thelength, C, of chamber 140/150 is close to the length, L, of coolingelement 120. For example, in some embodiments, the distance, d, betweenthe edge of cooling element 120 and the wall of chamber 140/150 is atleast one hundred micrometers and not more than five hundredmicrometers. In some embodiments, d is at least two hundred micrometersand not more than three hundred micrometers.

Cooling element 120 may be driven at a frequency that is at or near boththe resonant frequency for an acoustic resonance of a pressure wave ofthe fluid in top chamber 140 and the resonant frequency for a structuralresonance of cooling element 120. The portion of cooling element 120undergoing vibrational motion is driven at or near resonance (the“structural resonance”) of cooling element 120. This portion of coolingelement 120 undergoing vibration may be a cantilevered section in someembodiments. The frequency of vibration for structural resonance istermed the structural resonant frequency. Use of the structural resonantfrequency in driving cooling element 120 reduces the power consumptionof cooling system 100. Cooling element 120 and top chamber 140 may alsobe configured such that this structural resonant frequency correspondsto a resonance in a pressure wave in the fluid being driven through topchamber 140 (the acoustic resonance of top chamber 140). The frequencyof such a pressure wave is termed the acoustic resonant frequency. Atacoustic resonance, a node in pressure occurs near vent 112 and anantinode in pressure occurs near the periphery of cooling system 100(e.g. near tip 121 of cooling element 120 and near the connectionbetween top chamber 140 and bottom chamber 150). The distance betweenthese two regions is C/2. Thus, C/2=nλ/4, where λ is the acousticwavelength for the fluid and n is odd (e.g. n=1, 3, 5, etc.). For thelowest order mode, C=λ/2. Because the length of chamber 140 (e.g. C) isclose to the length of cooling element 120, in some embodiments, it isalso approximately true that L/2=nλ/4, where λ is the acousticwavelength for the fluid and n is odd. Thus, the frequency at whichcooling element 120 is driven, ν, is at or near the structural resonantfrequency for cooling element 120. The frequency ν is also at or nearthe acoustic resonant frequency for at least top chamber 140. Theacoustic resonant frequency of top chamber 140 generally varies lessdramatically with parameters such as temperature and size than thestructural resonant frequency of cooling element 120. Consequently, insome embodiments, cooling element 120 may be driven at (or closer to) astructural resonant frequency rather than to the acoustic resonantfrequency.

Orifice plate 130 has orifices 132 therein. Although a particular numberand distribution of orifices 132 are shown, another number and/oranother distribution may be used. A single orifice plate 130 is used fora single cooling system 100. In other embodiments, multiple coolingsystems 100 may share an orifice plate. For example, multiple cells 100may be provided together in a desired configuration. In suchembodiments, the cells 100 may be the same size and configuration ordifferent size(s) and/or configuration(s). Orifices 132 are shown ashaving an axis oriented normal to a surface of heat-generating structure102. In other embodiments, the axis of one or more orifices 132 may beat another angle. For example, the angle of the axis may be selectedfrom substantially zero degrees and a nonzero acute angle. Orifices 132also have sidewalls that are substantially parallel to the normal to thesurface of orifice plate 130. In some embodiments, orifices may havesidewalls at a nonzero angle to the normal to the surface of orificeplate 130. For example, orifices 132 may be cone-shaped. Further,although orifice place 130 is shown as substantially flat, in someembodiments, trenches and/or other structures may be provided in orificeplate 130 to modify the configuration of bottom chamber 150 and/or theregion between orifice plate 130 and heat-generating structure 102.

The size, distribution and locations of orifices 132 are chosen tocontrol the flow rate of fluid driven to the surface of heat-generatingstructure 102. The locations and configurations of orifices 132 may beconfigured to increase/maximize the fluid flow from bottom chamber 150through orifices 132 to the jet channel (the region between the bottomof orifice plate 130 and the top of heat-generating structure 102). Thelocations and configurations of orifices 132 may also be selected toreduce/minimize the suction flow (e.g. back flow) from the jet channelthrough orifices 132. For example, the locations of orifices are desiredto be sufficiently far from tip 121 that suction in the upstroke ofcooling element 120 (tip 121 moves away from orifice plate 130) thatwould pull fluid into bottom chamber 150 through orifices 132 isreduced. The locations of orifices are also desired to be sufficientlyclose to tip 121 that suction in the upstroke of cooling element 120also allows a higher pressure from top chamber 140 to push fluid fromtop chamber 140 into bottom chamber 150. In some embodiments, the ratioof the flow rate from top chamber 140 into bottom chamber 150 to theflow rate from the jet channel through orifices 132 in the upstroke (the“net flow ratio”) is greater than 2:1. In some embodiments, the net flowratio is at least 85:15. In some embodiments, the net flow ratio is atleast 90:10. In order to provide the desired pressure, flow rate,suction, and net flow ratio, orifices 132 are desired to be at least adistance, r1, from tip 121 and not more than a distance, r2, from tip121 of cooling element 120. In some embodiments, r1 is at least onehundred micrometers (e.g. r1≥100 μm) and r2 is not more than onemillimeter (e.g. r2≤1000 μm). In some embodiments, orifices 132 are atleast two hundred micrometers from tip 121 of cooling element 120 (e.g.r1≥200 μm). In some such embodiments, orifices 132 are at least threehundred micrometers from tip 121 of cooling element 120 (e.g. r1≥300μm). In some embodiments, orifices 132 have a width, o, of at least onehundred micrometers and not more than five hundred micrometers. In someembodiments, orifices 132 have a width of at least two hundredmicrometers and not more than three hundred micrometers. In someembodiments, the orifice separation, s, is at least one hundredmicrometers and not more than one millimeter. In some such embodiments,the orifice separation is at least four hundred micrometers and not morethan six hundred micrometers. In some embodiments, orifices 132 are alsodesired to occupy a particular fraction of the area of orifice plate130. For example, orifices 132 may cover at least five percent and notmore than fifteen percent of the footprint of orifice plate 130 in orderto achieve a desired flow rate of fluid through orifices 132. In someembodiments, orifices 132 cover at least eight percent and not more thantwelve percent of the footprint of orifice plate 130.

In some embodiments, cooling element 120 is actuated using apiezoelectric. Thus, cooling element 120 may be a piezoelectric coolingelement. Cooling element 120 may be driven by a piezoelectric that ismounted on or integrated into cooling element 120. In some embodiments,cooling element 120 is driven in another manner including but notlimited to providing a piezoelectric on another structure in coolingsystem 100. Cooling element 120 and analogous cooling elements arereferred to hereinafter as piezoelectric cooling elements though it ispossible that a mechanism other than a piezoelectric might be used todrive the cooling element. In some embodiments, cooling element 120includes a piezoelectric layer on substrate. The substrate may includeor consist of stainless steel, a Ni alloy, Hastelloy, Al (e.g. an Alalloy), and/or Ti (e.g. a Ti alloy such as Ti6Al-4V). In someembodiments, a piezoelectric layer includes multiple sublayers formed asthin films on the substrate. In other embodiments, the piezoelectriclayer may be a bulk layer affixed to the substrate. Such a piezoelectriccooling element 120 also includes electrodes used to activate thepiezoelectric. The substrate functions as an electrode in someembodiments. In other embodiments, a bottom electrode may be providedbetween the substrate and the piezoelectric layer. Other layersincluding but not limited to seed, capping, passivation, or other layersmight be included in the piezoelectric cooling element. Thus, coolingelement 120 may be actuated using a piezoelectric.

In some embodiments, cooling system 100 includes chimneys (not shown) orother ducting. Such ducting provides a path for heated fluid to flowaway from heat-generating structure 102. In some embodiments, ductingreturns fluid to the side of top plate 110 distal from heat-generatingstructure 102. In some embodiments, ducting may instead direct fluidaway from heat-generating structure 102 in a direction parallel toheat-generating structure 102 or perpendicular to heat-generatingstructure 102 but in the opposite direction (e.g. toward the bottom ofthe page). For a device in which fluid external to the device is used incooling system 100, the ducting may channel the heated fluid to a vent.In such embodiments, additional fluid may be provided from an inletvent. In embodiments, in which the device is enclosed, the ducting mayprovide a circuitous path back to the region near vent 112 and distalfrom heat-generating structure 102. Such a path allows for the fluid todissipate heat before being reused to cool heat-generating structure102. In other embodiments, ducting may be omitted or configured inanother manner. Thus, the fluid is allowed to carry away heat fromheat-generating structure 102.

Operation of cooling system 100 is described in the context of FIGS.1A-1F. Although described in the context of particular pressures, gapsizes, and timing of flow, operation of cooling system 100 is notdependent upon the explanation herein. FIGS. 1C-1D depict in-phaseoperation of cooling system 100. Referring to FIG. 1C, cooling element120 has been actuated so that its tip 121 moves away from top plate 110.FIG. 1C can thus be considered to depict the end of a down stroke ofcooling element 120. Because of the vibrational motion of coolingelement 120, gap 152 for bottom chamber 150 has decreased in size and isshown as gap 152B. Conversely, gap 142 for top chamber 140 has increasedin size and is shown as gap 142B. During the down stroke, a lower (e.g.minimum) pressure is developed at the periphery when cooling element 120is at the neutral position. As the down stroke continues, bottom chamber150 decreases in size and top chamber 140 increases in size as shown inFIG. 1C. Thus, fluid is driven out of orifices 132 in a direction thatis at or near perpendicular to the surface of orifice plate 130 and/orthe top surface of heat-generating structure 102. The fluid is drivenfrom orifices 132 toward heat-generating structure 102 at a high speed,for example in excess of thirty-five meters per second. In someembodiments, the fluid then travels along the surface of heat-generatingstructure 102 and toward the periphery of heat-generating structure 102,where the pressure is lower than near orifices 132. Also in the downstroke, top chamber 140 increases in size and a lower pressure ispresent in top chamber 140. As a result, fluid is drawn into top chamber140 through vent 112. The motion of the fluid into vent 112, throughorifices 132, and along the surface of heat-generating structure 102 isshown by unlabeled arrows in FIG. 1C.

Cooling element 120 is also actuated so that tip 121 moves away fromheat-generating structure 102 and toward top plate 110. FIG. 1D can thusbe considered to depict the end of an up stroke of cooling element 120.Because of the motion of cooling element 120, gap 142 has decreased insize and is shown as gap 142C. Gap 152 has increased in size and isshown as gap 152C. During the upstroke, a higher (e.g. maximum) pressureis developed at the periphery when cooling element 120 is at the neutralposition. As the upstroke continues, bottom chamber 150 increases insize and top chamber 140 decreases in size as shown in FIG. 1D. Thus,the fluid is driven from top chamber 140 (e.g. the periphery of chamber140/150) to bottom chamber 150. Thus, when tip 121 of cooling element120 moves up, top chamber 140 serves as a nozzle for the entering fluidto speed up and be driven towards bottom chamber 150. The motion of thefluid into bottom chamber 150 is shown by unlabeled arrows in FIG. 1D.The location and configuration of cooling element 120 and orifices 132are selected to reduce suction and, therefore, back flow of fluid fromthe jet channel (between heat-generating structure 102 and orifice plate130) into orifices 132 during the upstroke. Thus, cooling system 100 isable to drive fluid from top chamber 140 to bottom chamber 150 withoutan undue amount of backflow of heated fluid from the jet channelentering bottom chamber 150. Moreover, cooling system 100 may operatesuch that fluid is drawn in through vent 112 and driven out throughorifices 132 without cooling element 120 contacting top plate 110 ororifice plate 130. Thus, pressures are developed within chambers 140 and150 that effectively open and close vent 112 and orifices 132 such thatfluid is driven through cooling system 100 as described herein.

The motion between the positions shown in FIGS. 1C and 1D is repeated.Thus, cooling element 120 undergoes vibrational motion indicated inFIGS. 1A-1D, drawing fluid through vent 112 from the distal side of topplate 110 into top chamber 140; transferring fluid from top chamber 140to bottom chamber 150; and pushing the fluid through orifices 132 andtoward heat-generating structure 102. As discussed above, coolingelement 120 is driven to vibrate at or near the structural resonantfrequency of cooling element 120. Further, the structural resonantfrequency of cooling element 120 is configured to align with theacoustic resonance of the chamber 140/150. The structural and acousticresonant frequencies are generally chosen to be in the ultrasonic range.For example, the vibrational motion of cooling element 120 may be atfrequencies from 15 kHz through 30 kHz. In some embodiments, coolingelement 120 vibrates at a frequency/frequencies of at least 20 kHz andnot more than 30 kHz. The structural resonant frequency of coolingelement 120 is within ten percent of the acoustic resonant frequency ofcooling system 100. In some embodiments, the structural resonantfrequency of cooling element 120 is within five percent of the acousticresonant frequency of cooling system 100. In some embodiments, thestructural resonant frequency of cooling element 120 is within threepercent of the acoustic resonant frequency of cooling system 100.Consequently, efficiency and flow rate may be enhanced. However, otherfrequencies may be used.

Fluid driven toward heat-generating structure 102 may move substantiallynormal (perpendicular) to the top surface of heat-generating structure102. In some embodiments, the fluid motion may have a nonzero acuteangle with respect to the normal to the top surface of heat-generatingstructure 102. In either case, the fluid may thin and/or form aperturesin the boundary layer of fluid at heat-generating structure 102. As aresult, transfer of heat from heat-generating structure 102 may beimproved. The fluid deflects off of heat-generating structure 102,traveling along the surface of heat-generating structure 102. In someembodiments, the fluid moves in a direction substantially parallel tothe top of heat-generating structure 102. Thus, heat fromheat-generating structure 102 may be extracted by the fluid. The fluidmay exit the region between orifice plate 130 and heat-generatingstructure 102 at the edges of cooling system 100. Chimneys or otherducting (not shown) at the edges of cooling system 100 allow fluid to becarried away from heat-generating structure 102. In other embodiments,heated fluid may be transferred further from heat-generating structure102 in another manner. The fluid may exchange the heat transferred fromheat-generating structure 102 to another structure or to the ambientenvironment. Thus, fluid at the distal side of top plate 110 may remainrelatively cool, allowing for the additional extraction of heat. In someembodiments, fluid is circulated, returning to the distal side of topplate 110 after cooling. In other embodiments, heated fluid is carriedaway and replaced by new fluid at the distal side of cooling element120. As a result, heat-generating structure 102 may be cooled.

FIGS. 1E-1F depict an embodiment of active MEMS cooling system 100including centrally anchored cooling element 120 in which the coolingelement is driven out-of-phase. More specifically, sections of coolingelement 120 on opposite sides of anchor 160 (and thus on opposite sidesof the central region of cooling element 120 that is supported by anchor160) are driven to vibrate out-of-phase. In some embodiments, sectionsof cooling element 120 on opposite sides of anchor 160 are driven at ornear one hundred and eighty degrees out-of-phase. Thus, one section ofcooling element 120 vibrates toward top plate 110, while the othersection of cooling element 120 vibrates toward orifice plate130/heat-generating structure 102. Movement of a section of coolingelement 120 toward top plate 110 (an upstroke) drives fluid in topchamber 140 to bottom chamber 150 on that side of anchor 160. Movementof a section of cooling element 120 toward orifice plate 130 drivesfluid through orifices 132 and toward heat-generating structure 102.Thus, fluid traveling at high speeds (e.g. speeds described with respectto in-phase operation) is alternately driven out of orifices 132 onopposing sides of anchor 160. Because fluid is driven through orifices132 at high speeds, cooling system 100 may be viewed as a MEMs jet. Themovement of fluid is shown by unlabeled arrows in FIGS. 1E and 1F.Themotion between the positions shown in FIGS. 1E and 1F is repeated. Thus,cooling element 120 undergoes vibrational motion indicated in FIGS. 1A,1E, and 1F, alternately drawing fluid through vent 112 from the distalside of top plate 110 into top chamber 140 for each side of coolingelement 120; transferring fluid from each side of top chamber 140 to thecorresponding side of bottom chamber 150; and pushing the fluid throughorifices 132 on each side of anchor 160 and toward heat-generatingstructure 102. As discussed above, cooling element 120 is driven tovibrate at or near the structural resonant frequency of cooling element120. Further, the structural resonant frequency of cooling element 120is configured to align with the acoustic resonance of the chamber140/150. The structural and acoustic resonant frequencies are generallychosen to be in the ultrasonic range. For example, the vibrationalmotion of cooling element 120 may be at the frequencies described forin-phase vibration. The structural resonant frequency of cooling element120 is within ten percent of the acoustic resonant frequency of coolingsystem 100. In some embodiments, the structural resonant frequency ofcooling element 120 is within five percent of the acoustic resonantfrequency of cooling system 100. In some embodiments, the structuralresonant frequency of cooling element 120 is within three percent of theacoustic resonant frequency of cooling system 100. Consequently,efficiency and flow rate may be enhanced. However, other frequencies maybe used.

Fluid driven toward heat-generating structure 102 for out-of-phasevibration may move substantially normal (perpendicular) to the topsurface of heat-generating structure 102, in a manner analogous to thatdescribed above for in-phase operation. Similarly, chimneys or otherducting (not shown) at the edges of cooling system 100 allow fluid to becarried away from heat-generating structure 102. In other embodiments,heated fluid may be transferred further from heat-generating structure102 in another manner. The fluid may exchange the heat transferred fromheat-generating structure 102 to another structure or to the ambientenvironment. Thus, fluid at the distal side of top plate 110 may remainrelatively cool, allowing for the additional extraction of heat. In someembodiments, fluid is circulated, returning to the distal side of topplate 110 after cooling. In other embodiments, heated fluid is carriedaway and replaced by new fluid at the distal side of cooling element120. As a result, heat-generating structure 102 may be cooled.

Although shown in the context of a uniform cooling element in FIGS.1A-1F, cooling system 100 may utilize cooling elements having differentshapes. FIG. 1G depicts an embodiment of engineered cooling element 120′having a tailored geometry and usable in a cooling system such ascooling system 100. Cooling element 120′ includes an anchored region 122and cantilevered arms 123. Anchored region 122 is supported (e.g. heldin place) in cooling system 100 by anchor 160. Cantilevered arms 123undergo vibrational motion in response to cooling element 120′ beingactuated. Each cantilevered arm 123 includes step region 124, extensionregion 126 and outer region 128. In the embodiment shown in FIG. 1G,anchored region 122 is centrally located. Step region 124 extendsoutward from anchored region 122. Extension region 126 extends outwardfrom step region 124. Outer region 128 extends outward from extensionregion 126. In other embodiments, anchored region 122 may be at one edgeof the actuator and outer region 128 at the opposing edge. In suchembodiments, the actuator is edge anchored.

Extension region 126 has a thickness (extension thickness) that is lessthan the thickness of step region 124 (step thickness) and less than thethickness of outer region 128 (outer thickness). Thus, extension region126 may be viewed as recessed. Extension region 126 may also be seen asproviding a larger bottom chamber 150. In some embodiments, the outerthickness of outer region 128 is the same as the step thickness of stepregion 124. In some embodiments, the outer thickness of outer region 128is different from the step thickness of step region 124. In someembodiments, outer region 128 and step region 124 each have a thicknessof at least three hundred twenty micrometers and not more than threehundred and sixty micrometers. In some embodiments, the outer thicknessis at least fifty micrometers and not more than two hundred micrometersthicker than the extension thickness. Stated differently, the step(difference in step thickness and extension thickness) is at least fiftymicrometers and not more than two hundred micrometers. In someembodiments, the outer step (difference in outer thickness and extensionthickness) is at least fifty micrometers and not more than two hundredmicrometers. Outer region 128 may have a width, o, of at least onehundred micrometers and not more than three hundred micrometers.Extension region 126 has a length, e, extending outward from the stepregion of at least 0.5 millimeter and not more than 1.5 millimeters insome embodiments. In some embodiments, outer region 128 has a highermass per unit length in the direction from anchored region 122 thanextension region 126. This difference in mass may be due to the largersize of outer region 128, a difference in density between portions ofcooling element 120, and/or another mechanism.

Use of engineered cooling element 120′ may further improve efficiency ofcooling system 100. Extension region 126 is thinner than step region 124and outer region 128. This results in a cavity in the bottom of coolingelement 120′ corresponding to extension region 126. The presence of thiscavity aids in improving the efficiency of cooling system 100. Eachcantilevered arm 123 vibrates towards top plate 110 in an upstroke andaway from top plate 110 in a downstroke. When a cantilevered arm 123moves toward top plate 110, higher pressure fluid in top chamber 140resists the motion of cantilevered arm 123. Furthermore, suction inbottom chamber 150 also resists the upward motion of cantilevered arm123 during the upstroke. In the downstroke of cantilevered arm 123,increased pressure in the bottom chamber 150 and suction in top chamber140 resist the downward motion of cantilevered arm 123. However, thepresence of the cavity in cantilevered arm 123 corresponding toextension region 126 mitigates the suction in bottom chamber 150 duringan upstroke. The cavity also reduces the increase in pressure in bottomchamber 150 during a downstroke. Because the suction and pressureincrease are reduced in magnitude, cantilevered arms 123 may morereadily move through the fluid. This may be achieved while substantiallymaintaining a higher pressure in top chamber 140, which drives the fluidflow through cooling system 100. Moreover, the presence of outer region128 may improve the ability of cantilevered arm 123 to move through thefluid being driven through cooling system 100. Outer region 128 has ahigher mass per unit length and thus a higher momentum. Consequently,outer region 128 may improve the ability of cantilevered arms 123 tomove through the fluid being driven through cooling system 100. Themagnitude of the deflection of cantilevered arm 123 may also beincreased. These benefits may be achieved while maintaining thestiffness of cantilevered arms 123 through the use of thicker stepregion 124. Further, the larger thickness of outer region 128 may aid inpinching off flow at the bottom of a downstroke. Thus, the ability ofcooling element 120′ to provide a valve preventing backflow throughorifices 132 may be improved. Thus, performance of cooling system 100employing cooling element 120′ may be improved.

Further, cooling elements used in cooling system 100 may have differentstructures and/or be mounted differently than depicted in FIGS. 1A-1G.In some embodiments, the cooling element may have rounded corners and/orrounded ends but still be anchored along a central axis such thatcantilevered arms vibrate. The cooling element may be anchored only atits central region such that the regions surrounding the anchor vibratein a manner analogous to a jellyfish or the opening/closing of anumbrella. In some such embodiments, the cooling element may be circularor elliptical in shape. In some embodiments, the anchor may includeapertures through which fluid may flow. Such an anchor may be utilizedfor the cooling element being anchored at its top (e.g. to the topplate). Although not indicated in FIGS. 1A-1G, the piezoelectricutilized in driving the cooling element may have various locationsand/or configurations. For example, the piezoelectric may be embedded inthe cooling element, affixed to one side of the cooling element (orcantilevered arm(s)), may occupy some or all of the cantilevered arms,and/or may have a location that is close to or distal from the anchoredregion. In some embodiments, cooling elements that are not centrallyanchored may be used. For example, a pair of cooling elements that haveoffset apertures, that are anchored at their ends (or all edges), andwhich vibrate out of phase may be used. Thus, various additionalconfigurations of cooling element 120 and/or 120′, anchor 160, and/orother portions of cooling system 100 may be used.

Using the cooling system 100 actuated for in-phase vibration orout-of-phase vibration of cooling element 120 and/or 120′, fluid drawnin through vent 112 and driven through orifices 132 may efficientlydissipate heat from heat-generating structure 102. Because fluidimpinges upon the heat-generating structure with sufficient speed (e.g.at least thirty meters per second) and in some embodiments substantiallynormal to the heat-generating structure, the boundary layer of fluid atthe heat-generating structure may be thinned and/or partially removed.Consequently, heat transfer between heat-generating structure 102 andthe moving fluid is improved. Because the heat-generating structure ismore efficiently cooled, the corresponding integrated circuit may be runat higher speed and/or power for longer times. For example, if theheat-generating structure corresponds to a high-speed processor, such aprocessor may be run for longer times before throttling. Thus,performance of a device utilizing cooling system 100 may be improved.Further, cooling system 100 may be a MEMS device. Consequently, coolingsystems 100 may be suitable for use in smaller and/or mobile devices,such as smart phones, other mobile phones, virtual reality headsets,tablets, two-in-one computers, wearables and handheld games, in whichlimited space is available. Performance of such devices may thus beimproved. Because cooling element 120/120′ may be vibrated atfrequencies of 15 kHz or more, users may not hear any noise associatedwith actuation of cooling elements. If driven at or near structuraland/or acoustic resonant frequencies, the power used in operatingcooling systems may be significantly reduced. Cooling element 120/120′does not physically contact top plate 110 or orifice plate 130 duringvibration. Thus, resonance of cooling element 120/120′ may be morereadily maintained. More specifically, physical contact between coolingelement 120/120′ and other structures disturbs the resonance conditionsfor cooling element 120/120′. Disturbing these conditions may drivecooling element 120/120′ out of resonance. Thus, additional power wouldneed to be used to maintain actuation of cooling element 120/120′.Further, the flow of fluid driven by cooling element 120/120′ maydecrease. These issues are avoided through the use of pressuredifferentials and fluid flow as discussed above. The benefits ofimproved, quiet cooling may be achieved with limited additional power.Further, out-of-phase vibration of cooling element 120/120′ allows theposition of the center of mass of cooling element 120/120′ to remainmore stable. Although a torque is exerted on cooling element 120/120′,the force due to the motion of the center of mass is reduced oreliminated. As a result, vibrations due to the motion of cooling element120/120′ may be reduced. Moreover, efficiency of cooling system 100 maybe improved through the use of out-of-phase vibrational motion for thetwo sides of cooling element 120/120′. Consequently, performance ofdevices incorporating the cooling system 100 may be improved. Further,cooling system 100 may be usable in other applications (e.g. with orwithout heat-generating structure 102) in which high fluid flows and/orvelocities are desired.

FIGS. 2A-2B depict an embodiment of active MEMS cooling system 200including a top centrally anchored cooling element. FIG. 2A depicts aside view of cooling system 200 in a neutral position. FIG. 2B depicts atop view of cooling system 200. FIGS. 2A-2B are not to scale. Forsimplicity, only portions of cooling system 200 are shown. Referring toFIGS. 2A-2B, cooling system 200 is analogous to cooling system 100.Consequently, analogous components have similar labels. For example,cooling system 200 is used in conjunction with heat-generating structure202, which is analogous to heat-generating structure 102.

Cooling system 200 includes top plate 210 having vents 212, coolingelement 220 having tip 221, orifice plate 230 including orifices 232,top chamber 240 having a gap, bottom chamber 250 having a gap, flowchamber 240/250, and anchor (i.e. support structure) 260 that areanalogous to top plate 110 having vent 112, cooling element 120 havingtip 121, orifice plate 130 including orifices 132, top chamber 140having gap 142, bottom chamber 150 having gap 152, flow chamber 140/150,and anchor (i.e. support structure) 160, respectively. Also shown ispedestal 290 that is analogous to pedestal 190. Thus, cooling element220 is centrally supported by anchor 260 such that at least a portion ofthe perimeter of cooling element 220 is free to vibrate. In someembodiments, anchor 260 extends along the axis of cooling element 220.In other embodiments, anchor 260 is only near the center portion ofcooling element 220. Although not explicitly labeled in FIGS. 2A and 2B,cooling element 220 includes an anchored region and cantilevered armsincluding step region, extension region and outer regions analogous toanchored region 122, cantilevered arms 123, step region 124, extensionregion 126 and outer region 128 of cooling element 120′. In someembodiments, cantilevered arms of cooling element 220 are drivenin-phase. In some embodiments, cantilevered arms of cooling element 220are driven out-of-phase. In some embodiments, a simple cooling element,such as cooling element 120, may be used.

Anchor 260 supports cooling element 220 from above. Thus, coolingelement 220 is suspended from anchor 260. Anchor 260 is suspended fromtop plate 210. Top plate 210 includes vent 213. Vents 212 on the sidesof anchor 260 provide a path for fluid to flow into sides of chamber240.

As discussed above with respect to cooling system 100, cooling element220 may be driven to vibrate at or near the structural resonantfrequency of cooling element 220. Further, the structural resonantfrequency of cooling element 220 may be configured to align with theacoustic resonance of the chamber 240/250. The structural and acousticresonant frequencies are generally chosen to be in the ultrasonic range.For example, the vibrational motion of cooling element 220 may be at thefrequencies described with respect to cooling system 100. Consequently,efficiency and flow rate may be enhanced. However, other frequencies maybe used.

Cooling system 200 operates in an analogous manner to cooling system100. Cooling system 200 thus shares the benefits of cooling system 100.Thus, performance of a device employing cooling system 200 may beimproved. In addition, suspending cooling element 220 from anchor 260may further enhance performance. In particular, vibrations in coolingsystem 200 that may affect other cooling cells (not shown) may bereduced. For example, less vibration may be induced in top plate 210 dueto the motion of cooling element 220. Consequently, cross talk betweencooling system 200 and other cooling systems (e.g. other cells) or otherportions of the device incorporating cooling system 200 may be reduced.Thus, performance may be further enhanced.

FIGS. 3A-3E depict an embodiment of active MEMS cooling system 300including multiple cooling cells configured as a module termed a tile,or array. FIG. 3A depicts a perspective view, while FIGS. 3B-3E depictside views. FIGS. 3A-3E are not to scale. Cooling system 300 includesfour cooling cells 301A, 301B, 301C and 301D (collectively orgenerically 301), which are analogous to one or more of cooling systemsdescribed herein. More specifically, cooling cells 301 are analogous tocooling system 100 and/or 200. Tile 300 thus includes four cooling cells301 (i.e. four MEMS jets). Although four cooling cells 301 in a 2×2configuration are shown, in some embodiments another number and/oranother configuration of cooling cells 301 might be employed. In theembodiment shown, cooling cells 301 include shared top plate 310 havingapertures 312, cooling elements 320, shared orifice plate 330 includingorifices 332, top chambers 340, bottom chambers 350, anchors (supportstructures) 360, and pedestals 390 that are analogous to top plate 110having apertures 112, cooling element 120, orifice plate 130 havingorifices 132, top chamber 140, bottom chamber 150, anchor 160, andpedestal 190. In some embodiments, cooling cells 301 may be fabricatedtogether and separated, for example by cutting through top plate 310,side walls between cooling cells 301, and orifice plate 330. Thus,although described in the context of a shared top plate 310 and sharedorifice plate 330, after fabrication cooling cells 301 may be separated.In some embodiments, tabs (not shown) and/or other structures such asanchors 360 may connect cooling cells 301. Further, tile 300 includesheat-generating structure (termed a heat spreader hereinafter) 302 (e.g.a heat sink, a heat spreader, and/or other structure) that also hassidewalls, or fencing, in the embodiment shown. Cover plate 306 havingapertures therein is also shown. Heat spreader 302 and cover plate 306may be part of an integrated tile 300 as shown or may be separate fromtile 300 in other embodiments. Heat spreader 302 and cover plate 306 maydirect fluid flow outside of cooling cells 301, provide mechanicalstability, and/or provide protection. Electrical connection to coolingcells 301 is provided via flex connector 380 (not shown in FIGS. 3B-5E)which may house drive electronics 385. Cooling elements 320 are drivenout-of-phase (i.e. in a manner analogous to a seesaw). Further, as canbe seen in FIGS. 3B-3C and FIGS. 3D-3E cooling element 320 in one cellis driven out-of-phase with cooling element(s) 320 in adjacent cell(s).In FIGS. 3B-3C, cooling elements 320 in a row are driven out-of-phase.Thus, cooling element 320 in cell 301A is out-of-phase with coolingelement 320 in cell 301B. Similarly, cooling element 320 in cell 301C isout-of-phase with cooling element 320 in cell 301D. In FIGS. 3D-3E,cooling elements 320 in a column are driven out-of-phase. Thus, coolingelement 320 in cell 301A is out-of-phase with cooling element 320 incell 301C. Similarly, cooling element 320 in cell 301B is out-of-phasewith cooling element 320 in cell 301D. By driving cooling elements 320out-of-phase, vibrations in cooling system 300 may be reduced. Coolingelements 320 may be driven in another manner in some embodiments.

Cooling cells 301 of cooling system 300 function in an analogous mannerto cooling system(s) 100, 200, and/or an analogous cooling system.Consequently, the benefits described herein may be shared by coolingsystem 300. Because cooling elements in nearby cells are drivenout-of-phase, vibrations in cooling system 300 may be reduced. Becausemultiple cooling cells 301 are used, cooling system 300 may enjoyenhanced cooling capabilities. Further, multiples of individual coolingcells 301 and/or cooling system 300 may be combined in various fashionsto obtain the desired footprint of cooling cells.

FIGS. 4A-4D depict embodiments of active MEMS cooling systems 400A,400B, 400C, and 400D including a centrally anchored cooling element 420.FIGS. 4A and 4B are not to scale and for clarity, only some structuresare shown. Cooling system(s) 400A, 400B, 400C, and/or 400D are analogousto cooling systems 100 and/or 200. Cooling system(s) 400A, 400B, 400C,and/or 400D may be cooling cell(s) that are part of a cooling systemincluding multiple cells, such as tile 300. The device cooled by MEMScooling system(s) 400A, 400B, 400C, and/or 400D may be a laptopcomputer, a tablet or notebook computer, a smart phone, and/or othermobile devices. The device may also be another device, such as a serverin a rack, a game console, or a desktop computer. In some embodiments,therefore, the device is thin. For example, in some embodiments, thedevice in which cooling system(s) 400A, 400B, 400C, and/or 400D areintegrated has a thickness (height along the smallest dimension, thez-direction in FIGS. 4A-4D) of not more than twenty-five millimeters.The thickness is not more than ten millimeters in some embodiments. Insome such embodiments, the thickness of the device is not more thaneight millimeters. However, other thicknesses are possible. Further, thetotal height of cooling systems 400A, 400B, 400C, and/or 400D (e.g. fromthe top of heat-generating structure to the top of top plate 410) may beless than three millimeters.

Each cooling system 400A, 400B, 400C, and/or 400D includes top plate 410having vent(s) 412 therein, cooling element 420 having cantilevered arms423 and tip 429, top chamber 440, bottom chamber 450, and anchor 460that are analogous to top plate 110 having vent(s) 112 therein, coolingelement 120′, top chamber 140, bottom chamber 150, and anchor 160. Eachcooling system 400A, 400B, 400C, and 400D operates in a manner analogousto cooling system 100, 200, and/or 300.

Cooling element 420 is centrally supported by anchor 460 such that atleast a portion of the perimeter of cooling element 420 is free tovibrate. In the embodiment depicted, anchor 460 extends along the axisof cooling element 420 (i.e. along the y-direction). In otherembodiments, anchor 460 is only near the center portion of coolingelement 420. Although termed an “anchor,” in some embodiments, anchor460 may flex or rotate through small angles (e.g. less than one degree).Cooling element 420 includes actuator 422 and piezoelectrics 424.Piezoelectrics 424 are used to drive cooling element 420 to vibrate. Inthe embodiment shown, piezoelectrics 424 are on the upper surface(distal from bottom plate 430) of actuator 424. In some embodiments,piezoelectrics 424 may have another location and/or be integrated inanother manner. Cooling element 420 may also be considered to be dividedinto an anchored region 421 and cantilevered arms 423. Cantilevered arms423 of cooling element 420 may be driven in-Attorney phase. In someembodiments, cantilevered arms 423 of cooling element 420 are drivenout-of-phase.

Actuator 422 may be formed of materials having a high acoustic velocity(e.g. greater than four thousand meters per second) and low internallosses (e.g. not more than 0.1 percent structural losses). Examples ofmaterials that may be used for actuator 422 include stainless steel suchas SUS304 and/or SUS316, Al7075, Al6063, Ti64, Ti Grade 2, Ti Grade 9,Monel, aluminum bronze, and/or aluminum. In some embodiments, theactuator consists of one or more of the low internal loss materials. Insome embodiments, actuator 422 includes or consists of one or more ofSUS304, SUS316, Al7075, Al6063, Ti64, Ti Grade 2, and Ti Grade 9. Theuse of such low loss materials may improve the efficiency of coolingsystems 400A, 400B, 400C, and 400D.

Further, cooling element 420 (i.e. actuator 422 in the embodimentsshown) has bottom cavity 426-1 and top cavity 426-2. Cavities 426-1 and426-2 are indicated by dotted lines. In some embodiments, cavity 426-1does not extend to the edges of cooling element 420. In someembodiments, cavity 426-1 extends to the edges of cooling element 420.In such embodiments, cooling element 420 may be analogous to coolingelement 120′. In some embodiments, cavity 426-2 extends to the edges ofcooling element 420. In other embodiments, cavity 426-2 does not extendto the edges of cooling element 420. In some embodiments, cavity 426-1and/or 426-2 may be omitted. In some embodiments, another coolingelement, including but not limited to a simple cooling element such ascooling element 120, may be used in lieu of cooling element 420.

Cooling systems 400A, 400B, 400C, and 400D include bottom plates 431A,431B, 431C, and 431D. Bottom plates 431A, 431B, 431C, and 431D mayinclude or be formed of compliant materials that may have low internallosses. The low internal losses are not more than 0.1 percent losses insome embodiments. The material(s) used for bottom plates 431A, 431B,431C, and/or 431D may include at least one of Al7075, Al6063, Ti64 (TiGrade 5), Ti Grade 2, Ti Grade 9, a beryllium-copper alloy, Monel,aluminum bronze, aluminum, SUS304, or SUS316.

Bottom plates 431A, 431B, 431C, and 431D each include orifice plate430A, 430B, 430C, and 430D, respectively, and jet channel plate 435A,435B, 435C, and 435D, respectively. In some embodiments, orificeplate(s) 430A, 430B, 430C, and 430D and jet channel plate(s) 435A, 435B,435C, and 435D may be integrated into a single, monolithic bottom plate.Although depicted as part of jet channel plates 435A, 435B, 435C, and435D, a pedestal (indicated by dotted lines in the central portion ofeach jet channel plate 435A, 435B, 435C, and 435D) may be a separatestructure or may be omitted. In some embodiments, each orifice plate430A, 430B, 430C, and 430D is at least two hundred micrometers thick andnot more than four hundred micrometers thick (e.g. nominally threehundred micrometers thick). In some embodiments, each jet channel plate435A, 435B, 435C, and 435D is at least six hundred micrometers thick andnot more than eight hundred micrometers thick (e.g. nominally sevenhundred micrometers thick).

Also shown are heat-generating structures 402A, 402B, 402C, and 402D.Heat-generating structure(s) 402A, 420B, 402C, and/or 402D may be astructure such as an integrated circuit that may generate heat and isdesired to be cooled. More generally, heat-generating structures 402A,420B, 402C, and/or 402D may be a heat spreader (e.g. heat spreader 302)or analogous structure that is thermally coupled to a structure that isdesired to be cooled.

Referring to FIG. 4A, orifice plate 430A includes orifices 432A therein.Orifice plate 430A is analogous to orifice plate 130. Also shown inorifice plate 430A is edge 433. Thus, orifice plate 430A may be viewedas having a trench forming part of bottom cavity 450. Jet channel plate435A includes cavities 434A therein. In some embodiments, each cavity434A may have a width that is at least one-tenth and not more thanthree-eighths of the cooling cell width (e.g. width, C, depicted in FIG.1A). For example, cavities 434A may have a width that is at leastone-eighth and not more than three-eighths of the cooling cell width.

Orifice plate 430A and jet channel plate 435A are configured to set thedistance between orifices 432A and the top surface of heat-generatingstructure 402A. For bottom plate 431A, orifices 432A are a distance z1from the top of heat-generating structure 402A. The distance z1 is thethickness of jet channel plate 435A added to the height of gap 439A(also termed a jet channel herein) between jet channel plate 435A andheat-generating structure 402A.

Referring to FIG. 4B, jet channel plate 435B includes cavities 434B-1therein. Cavities 434B-1 are analogous to cavities 434A. However, jetchannel plate 435B also includes orifices 432B therein. Orifice plate430B includes cavities 434B-2 therein. In the embodiment shown, cavities434B-2 terminate at the top surface of jet channel plate 435B. In someembodiments, cavities 434B-2 extend into jet channel plate 435B. Thus,cavities 434B-1 are on a first side of orifices 432B, closer toheat-generating structure 402B. Cavities 434B-2 are on a second side oforifices 432B, further from heat-generating structure 402B. In someembodiments, each cavity 434B-1 and/or 434B-2 may have a width that isat least one-tenth and not more than three-eighths of the cooling cellwidth. For example, cavities 434B-1 and/or 434B-2 may have a width thatis at least one-eighth and not more than three-eighths of the coolingcell width. In some embodiments, cavities 434B-1 and 434B-2 have thesame width. In some embodiments, cavities 434B-1 and 434B-2 havedifferent widths. Orifice plate 430B and jet channel plate 435B areconfigured to set the distance between orifices 432B and the top surfaceof heat-generating structure 402B. Orifices 432B are a distance, z2,equal to the height of gap 439B between jet channel plate 435B andheat-generating structure 402B added to the depth of cavities 434B-1.For bottom plate 431B, orifices 432B are a distance z2 (z2<z1) from thetop of heat-generating structure 402B.

Referring to FIG. 4C, jet channel plate 435C includes orifices 432Ctherein. Cavities 434C extend through orifice plate 430C and partiallythrough jet channel plate 435C. The portions of cavities 434C in orificeplate 430C are shown as having the same width as the portions ofcavities 434C in jet channel plate 435C. In other embodiments, theseportions of cavities 434C may have different widths. In someembodiments, cavities 434C may have a width that is at least one-tenthand not more than three-eighths of the cooling cell width. For example,cavities 434C may have a width that is at least one-eighth and not morethan three-eighths of the cooling cell width. Orifice plate 430C and jetchannel plate 435C are configured to set the distance between orifices432C and the top surface of heat-generating structure 402C. For bottomplate 431C, orifices 432C are a distance z3 (z3<z2<z1) from the top ofheat-generating structure 402C. This distance z3 is the height of gap439C.

Referring to FIG. 4D, orifice plate 430D includes orifices 432D therein.Jet channel plate 435D includes cavities 434D. Thus, bottom plate 431Dis analogous to bottom plate 431A. However, heat-generating structure402D has depression 403 therein. Orifice plate 430D and jet channelplate 435D are configured to set the distance between orifices 432D andthe top surface of heat-generating structure 402D. For bottom plate431D, orifices 432D are a distance z4 (z3<z2<z1<z4) from the top ofheat-generating structure 402D.

By varying the configurations of bottom plates 431A, 431B, 431C, and431D and/or heat-generating structures 402A, 402B, 402C, and 402D, thedistance between orifices 432A, 432B, 432C, and 432D and the top surfaceof heat-generating structures 402A, 402B, 402C, and 402D (e.g. theportion of the surface substantially aligned with orifices 432A, 432B,432C, and 432D may be varied. Changing the distance between the orificesin a cooling system and the heat-generating structure may be used toconfigure the flow and/or other characteristics of the cooling system.In some embodiments, the distance between the orifices and theheat-generating structure may be at least two hundred micrometers.However, other distances are possible. For example, the distance betweenthe orifices and the heat-generating structure may be at least onehundred and fifty micrometers and not more than 1.5 millimeters. In someembodiments, the distance between the orifices and the heat-generatingstructure may be at least one hundred fifty and not more than fourhundred micrometers (e.g. may be nominally at least two hundredmicrometers and not more than three hundred micrometers). In someembodiments the distance between the orifices and the heat-generatingstructure may be at least five hundred micrometers and not more than onethousand micrometers (e.g. may be nominally 700 micrometers). In otherembodiments, the distance between the orifices and the heat-generatingstructure may be at least eight hundred micrometers and not more thanone thousand two hundred micrometers. Thus, configuration of thedistances between orifices and heat-generating structures may betailored.

Various distances between orifices 432A, 432B, 432C, and 432D andheat-generating structures 402A, 402B, 402C, and 402D are shown, forbottom plates 431A, 431B, 431C, and/or 431D. The lengths of orifices432A, 432B, 432C, and 432D may also be varied. For example, orifices432A may be lengthened by increasing the thickness of orifice plate430A. The jet channel plate configuration may also be changed to replacecavities with orifices. However, bottom plates 431A, 431B, 431C, and431D may have total thicknesses that are on the order of eight hundredmicrometers or more. In such embodiments, it may be undesirable fororifices to extend entirely through the orifice plate. Stateddifferently, orifices 432A, 432B, 432C, and 432D have a length that isless than eight hundred micrometers. Orifices 432A, 432B, 432C, and 432Dmay have a length that is less than eight hundred micrometers. In someembodiments, the orifice length may be at least one hundred and fiftymicrometers and not more than three hundred micrometers. In otherembodiments, the orifice length may be at least fifty micrometers andnot more than three hundred micrometers. In some embodiments, theorifice length may be at least seventy micrometers and not more than twohundred micrometers. In other embodiments, the orifice length may be atleast one hundred micrometers and not more than one hundred and fiftymicrometers. Such limitations on the lengths of orifices 432A, 432B,432C, and 432D may prevent narrowing of the peak in flow around theresonant frequencies of cooling systems 400A, 400B, 400C, and 400D.Thus, performance of such cooling systems 400A, 400B, 400C, and/or 400Dmay be improved.

FIGS. 5A-5B depict embodiments of active MEMS cooling systems 500A and500B including a centrally anchored cooling elements 520. FIGS. 5A and5B are not to scale and for clarity, only some structures are shown.Cooling system(s) 500A and/or 500B may be cooling cell(s) that are partof a cooling system including multiple cells, such as tile 300. Thedevice cooled by MEMS cooling system(s) 500A and/or 500B may be a laptopcomputer, a tablet or notebook computer, a smart phone, and/or othermobile devices. The device may also be another device, such as a serverin a rack, a game console, or a desktop computer. In some embodiments,therefore, the device is thin. For example, in some embodiments, thedevice in which cooling system(s) 500A and/or 500B are integrated has athickness (height along the smallest dimension, the z-direction in FIGS.5A-5D) of not more than twenty-five millimeters. The thickness is notmore than ten millimeters in some embodiments. In some such embodiments,the thickness of the device is not more than eight millimeters. However,other thicknesses are possible. Further, the total height of coolingsystem(s) 500A and/or 500B (e.g. from the top of heat-generatingstructures 502A and/or 502B to the top of top plate 510) may be lessthan three millimeters.

Cooling system(s) 500A and/or 500B are analogous to cooling systems400A, 400B, 400C, 400D, 100, and 200. Each cooling system 500A and/or500B includes top plate 510 having vent(s) 512 therein, cooling element520 having cantilevered arms 523, top chamber 540, bottom chamber 550,and anchor 560 that are analogous to top plate 410/110 having vent(s)412/112 therein, cooling element 420/120′, top chamber 440/140, bottomchamber 450/150, and anchor 460/160. Cooling element 520 has tip 529 andincludes actuator 522 and piezoelectrics 524 that are analogous to tip429, actuator 422, and piezoelectrics 424. Cooling element 520 may alsobe considered to be divided into anchored region 521 and cantileveredarms 523 that are analogous to anchored region 421 and cantilevered arms423. Cantilevered arms 523 may include cavities 526-1 and/or 526-2 thatare analogous to cavities 426-1 and/or 426-2. Cantilevered arms 523 ofcooling element 520 may be driven in-phase. In some embodiments,cantilevered arms 523 of cooling element 520 are driven out-of-phase.Further, the materials used for actuator 522 are analogous to those usedfor actuator 422 of cooling systems 400A, 400B, 400C, and/or 400D. Eachcooling system 500A and 500B operates in a manner analogous to coolingsystem(s) 100, 200, 300, and/or 400.

Cooling system(s) 500A and 500B also include bottom plates 531 that areanalogous to bottom plates 431A, 431B, 431C, and/or 431D. Bottom plate531 includes orifice plate 530 and jet channel plate 535 that areanalogous to orifices plates 430A, 430B, 430C, and/or 430D and jetchannel plates 435A, 435B, 435C, and/or 435D, respectively. Thus, jetchannel plate 535 includes cavities 534 that are analogous to cavities434A and 434D. In some embodiments, cavities 434B-1, 434B-2, and/or 434Cmay be present. Further, the materials used for bottom plates 531,orifice plates 530, and/or jet channel plates 535 are analogous to thoseused for the bottom plates, orifice plates, and/or jet channel plates ofcooling systems 400A, 400B, 400C, and/or 400D.

Also shown are heat-generating structures 502A and 502B. Heat-generatingstructures 502A and/or 502B may be structures such as an integratedcircuit that may generate heat and are desired to be cooled. Moregenerally, heat-generating structures 502A and/or 502B may be heatspreaders (e.g. heat spreader 302) or analogous structures that arethermally coupled to a structure that is desired to be cooled.Heat-generating structure 502A is analogous to heat-generatingstructures 402A, 402B, and/or 402C. Heat-generating structure 502B isanalogous to heat-generating structure 402D. Thus, heat-generatingstructure 502B has depression 503, which is analogous to depression 403.

Cooling system 500A is configured such that gap 539A is between thebottom of jet channel plate 535 and the top of heat-generating structure502A. Gap 539A has height g1. Cooling system 500B is configured suchthat gap 539B is between the bottom of jet channel plate 535 andheat-generating structure 502B. Gaps 539A and/or 539B may be configuredby changing the depths of depression 503, the height of the pedestalportion of jet channel plate 535, the manner in which jet channel plate535 is connected with heat-generating structure 502A and/or 502B, and/orusing another mechanism.

It has been determined that the deflection of actuator 522 for a givendriving voltage is generally lower for the heights of gaps 539A and/or539B decreasing toward zero and increasing for larger heights of gaps539A and/or 539B up to particular heights. Thus, for a particular gapheight range, actuator deflection and thus the flow of fluid throughcooling systems 500A and 500B may be higher. In some embodiments, thegap height (e.g. g1 and/or g2) is at least one hundred micrometers andnot more than six hundred micrometers. In some embodiments, the gapheight is at least two hundred and not more than five hundredmicrometers. For example, gap(s) 539A and/or 539B may be at least threehundred and not more than five hundred micrometers height. In someembodiments, gap(s) 539A and/or 539B is desired to be at least threehundred and fifty and not more than four hundred and fifty micrometers.Such an arrangement may have improved flow and, therefore, improvedcooling efficiency. It is believed that the improvement in flow in thisregime is due to an improved matching between the mechanical andacoustic resonance characteristics of the cooling cell. Consequently,gaps 539A and/or 539B between cooling system 500A and/or 500B (e.g. thedistance between jet channel plate 535 and heat-generating structure502A and/or 502B) and heat spreader 502A and/or 502B may be tailored forimproved performance.

Cooling systems 400A, 400B, 400C, 400D, 500A, and 500B are described inthe context of various features. The features of cooling systems 400A,400B, 400C, 400D, 500A, and/or 500B may be combined in various ways notexplicitly depicted. For example, orifices having different distances tothe heat-generating structures may be combined with a differentlyconfigured orifice plate and/or a heat-generating structure havingtrenches therein.

FIG. 6 depicts an embodiment of method 600 for using an active coolingsystem. Method 600 may include steps that are not depicted forsimplicity. Method 600 is described in the context of system 400.However, method 600 may be used with other cooling systems including butnot limited to systems and cells described herein.

A driving signal at a frequency and an input voltage corresponding tothe resonant state of one or more cooling elements is provided to theactive MEMS cooling system, at 602. In some embodiments, a drivingsignal having the frequency corresponding to the resonant frequency of aspecific cooling element is provided to that cooling element. In someembodiments, a driving signal is provided to multiple cooling elements.In such embodiments, the frequency of the driving signal corresponds tothe resonant state of one or more cooling elements being driven, astatistical measure of the resonance, and/or within a threshold of theresonance as discussed above.

Characteristic(s) of the MEMS cooling system are monitored while thecooling element(s) are driven to provide a feedback signal correspondingto a proximity to a resonant state of the cooling element(s), at 604. Insome embodiments, characteristic(s) of each individual cooling elementare monitored to determine the deviation of the frequency of vibrationfor that cooling element from the resonant frequency of that coolingelement. In some embodiments, characteristic(s) for multiple coolingelements are monitored at 604. The characteristic(s) monitored may be aproxy for resonance and/or a deviation therefrom. For example, thevoltage at the cooling element, the power drawn by the cooling element,power output by the power source, peak-to-peak current output by thepower source, peak voltage output by the power source, average currentoutput by the power source, RMS current output by the power source,average voltage output by the power source, amplitude of displacement ofthe at least one cooling element, RMS current through the coolingelement, peak voltage at the cooling element, average current throughthe at cooling element, average voltage at the at least one coolingelement, and/or the peak current drawn by the cooling element may bemonitored. Using the characteristic(s) monitored, a deviation from theresonant state of the cooling element (e.g. of the driving/vibrationfrequency the deviation from the resonant frequency) may be determined.

The frequency and/or input voltage is adjusted based on the feedbacksignal, at 606. More specifically, 606 includes updating the frequencyand/or input voltage, based on the feedback signal, to correspond toresonant state(s) of the cooling element(s) at 606. For example, thefrequency for the drive signal may be updated to more closely match theresonant frequency/frequencies. In some embodiments, updating thefrequency includes changing the frequency to correspond to a power drawncorresponding to the vibration of the cooling element(s) beingmaximized, a voltage provided at the cooling element(s) being maximized,a voltage across the cooling element(s) being minimized, and/or anamplitude of a current drawn by the at least one cooling element beingminimized. In some embodiments, 606 includes determining whether thefeedback signal indicates that a drift in the resonant frequency of thecooling element(s) exceeds a threshold and identifying a new frequencyin response to a determination that the drift exceeds the threshold. Thenew frequency accounts for the drift in the resonant frequency. Themethod also includes setting the new frequency as the frequency for thedriving signal in response to the new frequency being identified.

For example, cooling element 420 in MEMS cooling system 400A is driven,at 602. Thus, the cooling element 420 is driven at a frequency that isat or near resonance of the cooling element. Characteristics of coolingelement 420 within MEMS cooling system 400A are monitored, at 604. Thus,the drift of the cooling element(s) 420 from resonance may bedetermined. The frequency may be adjusted based on the monitoring of604, at 606. Thus, MEMS cooling systems 400A, 400B, 400C, and/or 400Dmay be kept at or near resonance.

Thus, using method 600, an active cooling system, such as coolingsystem(s) 100, 200, 300, 400A, 400B, 400C, 400D, 500A, and/or 500B usingcooling elements 120, 120′, 220, 320, 420, and/or 520 may be efficientlydriven. These cooling systems may also have improved performance due tothe configuration(s) of the bottom plate (e.g. orifice plate and/or jetchannel plate), the heat-generating structure, the spacing between theorifices and the heat-generating structure, the lengths of orifices,and/or the spacing between the bottom plate and the heat-generatingstructure. Thus, such cooling system may have further improvedefficiency and/or reliability. Thus, method 600 may be used to operateactive MEMS cooling systems and achieve the benefits described herein.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A cooling system, comprising: a supportstructure; and a cooling element having a thickness and including ananchored region and a cantilevered arm, the anchored region beingcoupled to and supported by the support structure, the cantilevered armextending outward from the anchored region wherein the cantilevered armincludes at least one cavity therein, the at least one cavity having adepth of at least one-third and not more than three-fourths of thethickness of the cooling element; wherein the cooling element isconfigured to undergo vibrational motion when actuated to drive a fluidfor cooling a heat-generating structure.
 2. The cooling system of claim1, wherein the depth of the at least one cavity is at least one-half andnot more than two-thirds of the thickness.
 3. The cooling system ofclaim 1 wherein the cantilevered arm includes a tip distal from theanchored region and wherein the at least one cavity includes a firstcavity on a bottom side of the cooling element and a second cavity on atop side of the cooling element, the first cavity being a distance fromthe tip and having a first depth, the second cavity having a seconddepth, the first depth plus the second depth being equal to the depth.4. The cooling system of claim 3, wherein the second cavity furtherincludes a recessed portion.
 5. The cooling system of claim 1, whereinthe at least one cavity of the cooling element further includes at leastone of an additional tab, a cavity extension and a cross rib.
 6. Thecooling system of claim 1, wherein the cantilevered arm includes a tipdistal from the anchored region, and wherein the at least one cavity isconfigured such that an anticlastic curvature across the tip is withinthirty percent of a maximum normalized deflection.
 7. The cooling systemof claim 6, wherein the anticlastic curvature is within twenty percentof the maximum normalized deflection.
 8. The cooling system of claim 1,wherein the cantilevered arm is configured such that at a maximumdeflection, a bend in the cantilevered arm is present in a region thatis at least one-half and not more than two-thirds along the cantileveredarm from the anchored region.
 9. The cooling system of claim 1, furthercomprising: a piezoelectric having a width, a length, and apiezoelectric thickness, the width being at least forty percent and notmore than fifty percent of a cooling element width, the length being atleast forty percent and not more than fifty percent of a cooling elementlength, the piezoelectric thickness being at least fifty micrometers andnot more than two hundred micrometers.
 10. The cooling system of claim1, further comprising: a top plate having at least one vent therein; abottom plate having a plurality of orifices therein, the bottom platebeing disposed between the cooling element and the heat-generatingstructure; and a plurality of cell walls, configured such that a topchamber is formed between the top plate and the cooling element and suchthat a bottom chamber is formed between the bottom plate, and thecooling element, the top chamber being in fluid communication with thebottom chamber.
 11. A cooling system, comprising: a plurality of coolingcells, each of the plurality of cooling cells including a coolingelement having a thickness and including an anchored region and acantilevered arm, the anchored region being coupled to and supported bya support structure, the cantilevered arm extending outward from theanchored region wherein the cantilevered arm includes at least onecavity therein, the at least one cavity having a depth of at leastone-third and not more than three-fourths of the thickness of thecooling element, the cooling element undergoing vibrational motion whenactuated to drive a fluid for cooling a heat-generating structure. 12.The cooling system of claim 11, wherein the depth of the at least onecavity is at least one-half and not more than two-thirds of thethickness.
 13. The cooling system of claim 11 wherein the cantileveredarm includes a tip distal from the anchored region and wherein the atleast one cavity includes a first cavity on a bottom side of the coolingelement and a second cavity on a top side of the cooling element, thefirst cavity being a distance from the tip and having a first depth, thesecond cavity having a second depth, the first depth plus the seconddepth being equal to the depth.
 14. The cooling system of claim 11,wherein the cantilevered arm includes a tip distal from the anchoredregion, and wherein the at least one cavity is configured such that ananticlastic curvature across the tip is within thirty percent of amaximum normalized deflection.
 15. The cooling system of claim 11,wherein the cantilevered arm is configured such that at a maximumdeflection, a bend in the cantilevered arm is present in a region thatis at least one-half and not more than two-thirds along the cantileveredarm from the anchored region.
 16. The cooling system of claim 11,further comprising: a piezoelectric having a width, a length, and apiezoelectric thickness, the width being at least forty percent and notmore than fifty percent of a cooling element width, the length being atleast forty percent and not more than fifty percent of a cooling elementlength, the piezoelectric thickness being at least fifty micrometers andnot more than two hundred micrometers
 17. A method of cooling aheat-generating structure, comprising: driving a cooling element toinduce a vibrational motion at a frequency, the cooling element having athickness and including an anchored region and a cantilevered arm, theanchored region being coupled to and supported by a support structure,the cantilevered arm extending outward from the anchored region whereinthe cantilevered arm includes at least one cavity therein, the at leastone cavity having a depth of at least one-third and not more thanthree-fourths of the thickness of the cooling element, the coolingelement being configured to undergo the vibrational motion when actuatedto drive a fluid for cooling the heat-generating structure.
 18. Themethod of claim 17 wherein the frequency corresponds to a resonantfrequency for the cantilevered arm.
 19. The method of claim 17, whereinthe depth of the at least one cavity is at least one-half and not morethan two-thirds of the thickness.
 20. The method of claim 17 wherein thecantilevered arm includes a tip distal from the anchored region andwherein the at least one cavity includes a first cavity on a bottom sideof the cooling element and a second cavity on a top side of the coolingelement, the first cavity being a distance from the tip and having afirst depth, the second cavity having a second depth, the first depthplus the second depth being equal to the depth.